Session Data Unit(SDU) is the session layer responsible for sending/receiving data from transport layer. See table below for OSI diagram. Generally increasing the value of SDU can result in improved performance, efficient network utilization when transmitting large amounts of data. However until Oracle 11g the SDU size was limited to 64K. Starting with Oracle12c, support for SDU is increased to 2M thereby allowing efficient use of high bandwidth network.
SDU can be set at database level by configuring DEFAULT_SDU_SIZE parameter in sqlnet.ora or SDU listener.ora parameter. On the client size SDU can set be by configuring DEFAULT_SDU_SIZE parameter in client sqlnet.ora or SDU tnsnames.ora parameter.
Note: SDU size actually used is the smaller of the client and server value.
Standard communication services and applications that everyone can use
Layer that takes data from application and presents it in a format that is recognized by other layers; Functions like character set conversion is done by this layer
Establishes, maintain & end connections
Delivers data without errors in sequence and no data loss or duplications. Large messages can be delivered but is constrained by message size limits imposed by the network layer.
Controls the way data will be sent to recipient ; Factors like the physical path the data should take based etc are performed by this layer. This layer translates logical addresses, or names, into physical addresses, performs routing
Handles the transfer of data across the network ; It establishes and terminates the logical link between two nodes.
The lowest layer of the OSI is generally concerned with the transfer of data medium like optical or electrical medium etc.